A buried structure having a semiconductor crystal as a current-blocking burying layer is used in a semiconductor optical device such as a semiconductor laser. This structure is very important in the practical use of a device because it has advantages such as reduction in threshold current of a semiconductor laser due to a good current-blocking, stabilization of an optical beam by transverse mode control, good heat dissipation from an active layer due to a semiconductor burying layer, and high long-term reliability.
A directly modulated laser which is one of components necessary for a large capacity optical transmission system is an important component as a light source of a middle/short-distance high speed optical transmission system. Thus, reduction in cost is strongly required for the directly modulated laser. More specifically, in order to reduce the cost, it becomes necessary to operate the directly modulated laser under an uncooled condition in which no cooling system such as a Peltier element is used in the mounting of the laser device, and improve fabrication yield. Consequently, the characteristics permitting the directly modulated laser to operate at a higher temperature and at a high speed are desired.
In order that the directly modulated laser may operate at the higher temperature and at the high speed, it is required to reduce a capacitance of the laser device and increase an light output power efficiency at a high temperature. As for the principles of an operation of this directly modulated laser, a light output power of the laser is directly modulated by modulating an injection current to the laser. A modulation speed is limited by a relaxation oscillation frequency and a device capacitance of the laser. The modulation speed increases as the relaxation oscillation frequency becomes higher. In order to increase the relaxation oscillation frequency, it is necessary to reduce lifetime of photons and increase a differential gain and photon density.
The semiconductor buried structure is roughly classified into buried structures using a high mesa and a low mesa. As for the buried structure using a high mesa, a lower cladding layer, an active layer, an upper cladding layer, and a contact layer are formed on a substrate successively. Moreover, a high stripe-like mesa which is about 2 μm in mesa width and about 3 μm in mesa height is formed by using a dielectric mask, and a current-blocking layer is buried in both sides of the mesa to be grown, thereby forming the buried structure using a high mesa.
On the other hand, as for the buried structure using a low mesa, a lower cladding layer, an active layer, and a part of an upper cladding layer are formed on a substrate successively, and a low stripe-like mesa which is about 2 μm in mesa width and about 1.5 μm in mesa height is formed by using a dielectric mask. Moreover, after a current-blocking layer is buried in the both sides of the mesa to be grown and the dielectric mask is then removed, an upper over-cladding layer and a contact layer are grown, thereby completing the buried structure using a low mesa. In order to enhance the light output power efficiency at the high temperature in the laser device such as the directly modulated semiconductor laser, the buried structure using a low mesa is suitable for attaining this rather than the buried structure using a high mesa. The reason for this is that a device resistance can be reduced since an area of an upper electrode obtained in the buried structure using a low mesa can take a larger value than that in the buried structure using a high mesa.
In addition, since the buried structure using a low mesa is low in mesa height, the burying growth for the buried structure using a low mesa is easier than that for the buried structure using a high mesa and thus the irregular crystal growth hardly occurs. As a result, it is possible to form the burying layer which is excellent in crystal quality. However, in case of the buried structure using a low mesa, in order to obtain the thickness of the burying layer required to sufficiently operate the current-blocking, the height of the surface of the burying layer becomes higher than that of the mesa. Thus, the surface after the burying growth is performed for the mesa has a concavo-convex shape. When the over-cladding layer and the contact layer are further grown on the surface having the concavo-convex shape, the concavo-convex shape remains on the contact layer as well. Though the over-cladding layer is ordinarily a binary semiconductor compound crystal, since the contact layer is ordinarily made of a semiconductor compound alloy crystal such a ternary or more complex alloy crystal, the composition change of the contact layer occurs. As a result, the lattice mismatch is generated between the over-cladding layer and the contact layer. Thus, there is encountered such a problem that the degradation of the crystal quality due to the strain is caused.
This problem causes the degradation of the in-plane fabrication yield and run-to-run reproducibility as well as the degradation of the device characteristics. Thus, the concavo-convex shape before the growth of the contact layer, i.e., after the growth of the over-cladding layer needs to be flattened so as not to cause a problem in the crystal quality of the contact layer.
In addition, the conductivity type of the substrate served to the fabrication of the semiconductor optical device exerts a large influence on the device characteristics. A lower substrate electrode able to obtain a large contact area is made of a p-type semiconductor whose contact resistance is larger than that of an n-type semiconductor, whereby the device resistance can be reduced and thus the device characteristics can be enhanced. When the substrate is a p-type substrate, the directly modulated semiconductor laser for which the high speed operation is required has an advantage of obtaining compatibility with an non-type transistor circuit which is excellent in high speed operation. Consequently, in the semiconductor laser device, especially, in the directly modulated laser, the buried structure using a low mesa on the p-type substrate is effective. Moreover, a device structure which has excellent device characteristics and with which the fabrication yield and the run-to-run reproducibility are enhanced, and a fabrication method thereof are essential to reduction in cost of the device.
[Patent document 1] U.S. Pat. No. 5,470,785
[Non-patent document 1] A. Dadgar et al, “Ruthenium: A superior compensator of InP”, Applied Physics Letters Vol. 73, No. 26, pp. 3878-3880, 1998
[Non-patent document 2] A. van Geelen et al, “Ruthenium doped higher power 1.48 μm SIPBH laser”, 11th International conference on Indium Phosphide and related materials Tubl-2, 1999